1. Field of the Invention
This Invention relates to an acoustic wave interfering device for generating, decoding and illuminating acoustic holograms and more particularly for the generation, decoding and illumination of moving acoustic holograms from the frequency, amplitude and phase distribution information present on conventional analogue records, tapes and CD's.
2. Prior Art
Holography as conventionally used is a kind of three-dimensional lensless photography that records the interference pattern created by two highly coherent beams. Conventional holograms are therefore fixed records of interference patterns. Their use involves a two-step process; first, the freezing of the interference pattern in-time and second, its unfreezing or illumination. In the past, film has been used as the recording medium but during the exposure period all movement must be kept well below one-thousandth of a millimeter or be completely smudged. Fixed film holograms are therefore difficult to make, the object to be photographed usually having to be placed on a several-ton block of stone. The further development of "motion holography", the recording and displaying of moving images by film holography has been problematical with only the simplest forms of animated film holograms having been demonstrated.
This invention show how "motion holography" can be done in the acoustic range without the use of film as a storage medium. Acoustic holography now provides a unique solution to the "motion problem" by replacing the fixed film record of an interference pattern with a "live" interference pattern which can be continuously altered by the information stored on a rotating disk.
In this new acoustic motion holography process in the first step or freezing, the generation of the 3-D interference pattern is delayed and only the one-dimensional information about the wave, its frequency, amplitude and phase is frozen in-time and recorded. The second step or unfreezing, done at a later time then requires the simultaneous generation, decoding and illumination of the "live" 3-D interference pattern by causing two identical sets of highly coherent one-dimensional waves to interfere while being simultaneously illuminated.
The "motion holography" process then is utilized as follows: for example, when an acoustic string instrument is plucked the emerging waves would be frozen one-dimensionally in-time in an analogue record, tape or CD. In the second step, at a later time upon the generation of the "live" 3-D interference pattern with its simultaneous illumination, the same emerging waves are unfrozen and the released waves would then continue to proceed out from the acoustic string instrument as if it were still actually present, while long removed.
The unfrozen acoustic waves are indistinguishable from the orginal waves and the observer hears to what all appearances is the "original live performance" in full three-dimensional form.
Acoustic holography then differs from photographic holography in that the interference pattern created by two highly coherent beams is not recorded on film but it is instead generated "live" from information previously stored on a revolving disk. This advantageously removes the user from the difficult process of recording the interference pattern.
However, acoustic holography still remains a two-step process requiring first that the one-dimensional information be frozen in-time on some medium and second that two highly coherent beams of energy by interferometry decodes the one-dimensional information while being simultaneously illuminated. For example, while the technology for freezing or recording the frequency, amplitude and phase distribution of a train of one-dimensional acoustic wave-fronts is long established such vinyl disk are not ordinarily viewed as "acoustic holograms" because the distance or phase information present cannot be unfrozen or decoded by conventional playback means which lacks step two.
Sterephonic sound then attempts to compensate for the lost of phase information. Such attempts however, do not address the "phase decoding problem" but instead substitutes a "stereophonic image" which does not compair favorably with the "live performance" in everyway such as a holographic or wavefront reconstructed B-D image would.
No device is known for generating, decoding and illuminating moving wave-front reconstructed 3-D acoustic holographic images from conventional analogue records, tapes and CD's.